This invention relates to a multilayer roll having a compliant polytetrafluoroethylene ("PTFE") top layer on an elastomeric base layer bonded to a rigid insert and the method of preparation of the roll which is particularly well suited for use in xerographic and electro photographic copying machines and printers.
There are a wide variety of rolls used in xerographic copiers and electro graphic printing devices. Typically, these are fabricated with a solid core or insert and have an intermediate elastomeric base layer covered by a smooth top coat, such as a fluoropolymer top coat. The top coat provides an outer surface area of low surface tension. In operations such as fusing, which are generally carried out at elevated temperatures, the low surface tension prevents toner from adhering to the roll surface, or reduced image quality and offset when toner attaches to the roll surface.
Among the various types of rolls utilized in these devices, include fuser, pressure and donor rolls, film forming and drying rolls in wet or liquid toner systems, corona rolls, squeegee rolls, photoconductor rolls, low friction rolls and printing rolls. Thus, there is a large need for rolls where the properties of the top coat can be varied to serve these various end uses. This includes rolls having improved release properties, extended useful life and protect the intermediate elastomeric base layer which is bonded to the roll insert. These needs arise particularly in the case of higher speed applications.
Articles made with PTFE over an elastomeric material are not new. A variety of products in the form of rollers, seals, o-rings baskets, sheets and fabrics are available using layers of PTFE and elastomeric materials. This combination is desirable in many applications because of the surface properties, inertness, chemical resistance of PTFE; and when constructed with an elastomeric material, it provides compliance to a product.
In fusing applications, compliance is an important attribute affecting the degree of fix, paper tracking and copy quality. In the art, it has been generally accepted that soft roll fusing provides superior copy quality compared to hard roll system. Through modifications of roll construction and toner chemistry, improvement in copy quality has been attained.
In order to retain the advantages of the elastomeric base coat such as good compliance and efficient heat transfer, it is desirable to provide as thin a PTFE top layer as possible. There are several ways in which a PTFE covered roll with an elastic base layer can be fabricated. Several of these are well known and used for producing rolls used in the electro photographic industry.
The most common way to prepare a roll having a fluoropolymer top layer on an elastomeric base layer is to heat shrink a pre-expanded fluoropolymer tube over a cured elastomer. A more elaborate method uses a "mold in place" technique. Here, an insert is centered in a PTFE or polytrifluorochloroethylene sleeve and elastomeric material is injected between the insert and the outer PTFE tube. In this method, the fluoropolymer sleeve is generally flexible, yet self-supporting and is typically between about 254 to 635 microns (15 and 25 mils) in thickness. In U.S. Pat. No. 3,613,168 to Rowland and Tabelle a flexible yet self-supporting relatively thick fluoropolymer sleeve is mounted on a spindle of an extruder to maintain the core and sleeve concentrically. Silicone rubber is then extruded into the space between the mounted tube and core, which has been mounted on the spindle so that the elastomer material bonds to the core and fluoropolymer sleeve after curing.
Other methods of manufacture include stretching a fluoropolymer sleeve to below its yield point and while at this stage, inserting an elastomeric covered roll into the stretched sleeve. Releasing the pressure on the sleeve allows the fluoropolymer sleeve to shrink back and come into complete contact with the roll. In this case, the diameter of the elastomeric roll is larger than the unstretched fluoropolymer sleeve. When a vacuum is used in this process, it provides the added advantage of an air free fluoropolymer/elastomer interface. Alternatively, a fluoropolymer powder or latex can be sprayed or coated onto an elastomeric base layer.
In European Patent Application EP 625 735 to Japan Gore-Tex rolls are provided with a release surface of a porous PTFE top coat in which the pores of the PTFE are impregnated with silicone rubber and placed over an elastic layer by a non-continuous adhesive layer. The entire assembly is then heated to fuse the adhesives and adhere the porous PTFE film to the silicone rubber.
Thin films of porous PTFE impregnated with uncured to silicone rubber for forming a release top coat are also described in European Patent No. EP 441 114 to Fuji Xerox Co., Ltd. and Japan Gore-Tex, Inc. Here, unbaked PTFE is expanded forming a fibrillated PTFE film with voids ranging from 30 to 98 percent and pores from 0.02 microns to 15 microns is soaked with one or more types of silicone rubber to fill the voids.
In U.S. Pat. No. 4,883,715 to Kuge, et al. and assigned to Canon an elastic rotatable member for image fixing having a resin layer is formed by applying an aqueous discussion of fluorine resin powder of PFA and PTFE on the roughened surface of an elastic layer, sintering above the crystalline meeting point of the resin and quickly cooling it. The patent describes that the sintered resin layer has crystallinity of not more than 95%, tensile strength of not less than 50 kg/cm.sup.2 and a contact angle in water of not less than 100 degrees. The completed roll is claimed to show the desirable rubber properties before application of the surface resin layer and the surface fluorine resin layer show the resin properties as if sintered alone.
While these rolls and methods of fabrication described in the prior art are generally suitable for producing rolls with PTFE top layers, it remains desirable to provide an improved roll having a compliant PTFE top layer and a method of fabricating such rolls.